Lactose tolerance is the ability to drink milk past infancy. Most mammals can't do this,[1][2] but, in a startling example of recent evolution, some populations of humans have evolved the beneficial mutation of lactase persistence, the continued activity of the enzymelactase into adulthood.

Most humans remain lactase non-persistent and hence suffer varying degrees of lactose intolerance (failure to digest milk accompanied by various undesirable gastrointestinal symptoms).[1] However, in areas where humans herd mammals, there are obvious advantages to being able to drink their copious milk. A mutation that appeared in Europe around 10,000 years ago[3][4] and a mutation that appeared separately in east Africa a mere 3-6,000 years ago each allowed humans to consume milk and products derived from it throughout their lives (convergent evolution).[5][6] Lactase persistence presents stronger selection pressure than any other known human gene.[7]

Creationists are not happy at such blatant and striking direct evidence of evolution in humans.

The lactase persistence haplotype experienced very strong positive selection[10] during the rise of dairy farming, around 5,000 to 10,000 years ago.[11] It can be found in 80% of European-descended humans. Lactase persistence also sprang up separately in Kenya around 3,000-6,000 years ago.[6] It appears the mutation sprung up fairly often, but only benefited from natural selection where it was advantageous, whereas it was eliminated by genetic drift or even actively selected against elsewhere.

Correspondingly, in places with a lack of dairy farming, lactase persistence is much rarer. It is very low in southeast Asia and sub-Saharan Africa, and is absent in the Bantu of South Africa and most Chinese populations[10] ...except amongst the nomads on the borders of China, who drank horse milk regularly (and horse milk is even higher in lactose than cow's milk). The nomads also make an alcoholic beverage, kumis, from mare milk.

The most important factor for lactose persistence is that cow or goat herders in the middle of a steppe have only a few possible sources of food: namely, their animals and whatever sparse vegetation may be fit for human consumption. Being able to consume milk and milk products is hugely beneficial and eliminates the need to slaughter an animal for every meal which could allow for a consistent food source during the entire time the animal produces milk. However, some herding nomads have also taken to drinking blood (in addition to milk) from their animals which they gather (mostly) in non-lethal ways.

Being able to drink milk throughout life turned out to be useful. In Northern Europe milk was a good source of Vitamin D, a shortage of which leads to the particularly nasty and painful disease rickets.[12] The 2009 British Women's Heart and Health Study reported more hip and wrist fractures, more osteoporosis and more cataracts in the lactase non-persistent group.[13] They also were on average very slightly shorter (4–6mm) than the other women, as well as weighing slightly less. These were after correcting for metabolic traits, socioeconomic status, lifestyle, and fertility.

Unsurprisingly, creationists had difficulty dealing with this incredibly blatant and powerful evidence of evolution, particularly when it hit the popular press.[5][14] The standard response was to claim that this evidence for evolution is evidence against evolution, on the basis that the failure to switch off lactase is a loss of information (in the special creationist sense of "information", which is biologically meaningless).[15]

The other response was to claim nothing is happening and hey, look over there:

"Lactase persistence has nothing to do with molecules-to-man evolution or ape-to-human evolution or even information-adding evolution. It is just a switched-on variation within the human genome."[16]

"If anything, the prevalence of lactase persistence is a testimony to the fact an all-knowingCreator designed the human genome with the ability to change."[15]